During injection molding, molten thermoplastic material flows into a mold cavity of a mold via one or more gates. The pressure within the mold cavity is an important vector, as insufficient pressure may result in improperly formed parts while excessive pressure may result in damage to the mold. The pressure at the melt flow front provides information relevant to the injection molding process overall by, for example, enabling calculation of how fast a mold cavity is filling and how long cooling of the molded part within the cavity is likely to take. Some injection molding processes aim to have the melt flow front follow a particular pressure pattern over time in order to optimize the injection molding process. For example, some injection molding processes maintain a pressure balance between the air pressure in the cavity, which changes based on the mold cavity geometry as the molten thermoplastic material moves into the cavity of the mold, and the pressure at the melt flow front, in order to produce a final product that is fully relieved of internal stresses that would otherwise undesirably lead to shrink, sink and warpage. For such injection molding systems, determining the real-time pressure at the melt flow front is important in order to determine whether the desired pressure/time curve is properly being followed and, if deviations from the pressure/time curve are identified, to make adjustments to correct the pressure of the melt flow front.
Ideally, sensors for measuring the pressure within a mold cavity and at a melt flow front would be indirect, easy to install, and inexpensive. Direct sensors, such as sensors placed within a mold cavity, leave undesirable marks on part surfaces. For example, while demand for injection molded parts with high gloss finishes has been increasing, direct sensors positioned in the mold cavity have a tendency to mar the high gloss finish of the parts. As a result, indirect sensors that are not located in the mold cavity are preferable. Some current indirect sensors include parting line sensors, ejector or static pin sensors, and ultrasonic sensors. Unfortunately, these indirect sensors cannot always be placed in optimal locations, sometimes require that a mold apparatus undergo a period of downtime in order to be machined so that the sensor can be mounted, and can be expensive. The same problems plague the current use of strain gauge sensors for indirectly measuring pressure within a mold cavity. For example, strain gauge sensors have been used in conjunction with molding apparatuses having ejector sleeves or long core pins, but not all injection molding apparatuses are configured to include an ejector sleeve or long core pin.